The consumer market for sensors is tending towards ever smaller component sizes in order to achieve prices that are competitive. However, measurement circuits for sensors are particularly complex because they require read electronics capable of detecting variations in impedances of dipole.
The impedances can be of the resistive type, as applies to piezoresistive strain gauges, and thermistors, of the capacitive type (variation in capacitance by variation in surface area or in airgap) or indeed of the inductive type. The measurement circuit needs to be configured to co-operate with passive sensors for microelectromechanical systems (MEMS) or nanoelectromechanical systems (NEMS), such as pressure sensors, temperature sensors, one-dimensional to three-dimensional accelerometers, one-dimensional to three-dimensional magnetometers, one-dimensional to three-dimensional gyrometers, and deformation or force sensors based on piezoresistive deformation gauges.
Passive sensors use various physical phenomena (piezoresistivity, photoresistivity, magnetoresistivity, variation in capacitance induced by variation in surface area or variation in airgap, etc.) to modify their impedances as a function of the value of the measurand that is to be evaluated (strain, deformation, displacement, light flux, magnetic field, etc.). Such sensors generally require a voltage or current source to bias the impedances of the dipole and thereby measure variation in them. For this type of circuit, it is necessary to use conditioners that encode the information, and more particularly to use potentiometer circuits.
In the state of the prior art, the solution that is most commonly used is the Wheatstone bridge. In the field of multi-axis sensors, a plurality of bridges need to be powered and read. In order to limit the number of terminals necessary for biasing and for reading, usually only a half Wheatstone bridge is formed by means of two variable impedances. The bridge is closed onto an electronic chip following the addition of two resistors having reference resistance.
For multi-axis reading, it is possible to share the power supply terminal. For such reading, it is possible to power and to read the bridges independently from one another. In certain situations, a single electronics circuit is put in place that has two functions: biasing and reading the bridges in successive manner.
However, one of the difficulties encountered for taking full advantage of the Wheatstone bridge is related to the connection parasitic resistances between the elements of the bridge and the external power supply circuits. The impact of the parasitic resistances can be minimized if their values are negligible compared with the resistances of the bridge for any given measurement precision. However, certain manufacturing techniques, such as using gold-silicon bonding (between the chip and its cap) can bring parasitic resistances of the same orders of magnitude as the resistances to be measured, such parasitic resistances not being known or controlled, and varying over time and with variations in temperature.
In such a situation, it is necessary to overcome such parasitic resistances so as not to disturb the measurement, in particular by using “four-point” or “four-terminal” connection techniques. The principle of that technique is to double the terminals serving for the power supply. The first terminal then serves to feed the current in. The current then causes a potential drop in the parasitic resistance, making the external potential different from the internal potential. However, that effect is of no consequence because the reading can then be taken on the second terminal. Since the reading involves only negligible currents, the internal and external potentials are the same.
In a “four-point” mode having one bridge, the measurement terminals are, in general, not doubled because the measurement electronics have very high impedance and the current flowing through them is very low. However, the voltage applied by the source cannot propagate fully to the bridge because of the voltage drop in the parasitic resistances. The number of terminals is thus doubled in order to be able to measure the voltage actually brought to the bridge. A read electronic circuit is necessary in order to measure the bias voltage.
In a second solution, the entire Wheatstone bridge has been placed at the MEMS chip and the number of terminals has been doubled, leading to a number of terminals that is prohibitive, in particular for a multi-axis sensor.
In addition, the document entitled “A Low-Voltage Current Mode Wheatstone Bridge using CMOS Transistors”, by Farshidi et al., International Journal of Electrical & Electronics Engineering, Issue 1, vol. 5, pages 38-42, relates to a measurement circuit including a Wheatstone bridge. That document makes provision for the circuit to have a current output. Analog converters, for example, work exclusively in voltage. Thus, in that type of circuit, it is necessary to add a current-to-voltage conversion stage in order to transform the current output into a voltage output. A major drawback is therefore the complexity of the circuit and the high cost of forming such a circuit.
The present invention makes it possible to solve all, or at least some, of the drawbacks generated by the Wheatstone bridge method. The invention also proposes a measurement circuit that is simple and inexpensive, and that serves as an alternative to the Wheatstone bridge, while also overcoming all the drawbacks hitherto encountered.